In general, in a control apparatus of an electric power steering apparatus, a detection is made by a torque sensor constructed by a steering torque potentiometer of a steering system, and an electric motor for producing steering auxiliary force to the above-described steering system is controlled by a control means in response to a torque detection value of the torque sensor.
As described above, in the control apparatus of the electric power steering apparatus, since the steering auxiliary force produced by the electric motor is controlled based on the torque detection value of the torque sensor, when the torque detection value detected by the torque sensor is brought into the abnormality condition, the correct control can be no longer performed. Therefore, it is required to detect the abnormality of the torque sensor.
Here, as the abnormality of the torque sensor, there are two types of abnormality. That is, the torque detection value is drifted due to variations in power supply voltages and the aging change in contact resistance values of a connector connected to the torque sensor. Also, the torque detection value becomes abnormal, because of abnormality in the torque sensor caused by variations in the torque detection value by the aging change in the contact resistance values of the connector, and also loose contacts of the sliding contactor of the potentiometer.
Conventionally, as the drift detecting circuit for detecting drift abnormality of the torque sensor, for instance, one drift detecting circuit as shown in FIG. 20 has been proposed.
In this prior art, a torque sensor 101 is so arranged that applied steering torque is converted into torsion angle displacement of a torsion bar, and this torsion angle displacement is detected by a main potentiometer 102 and a sub-potentiometer 103 series-connected to the main potentiometer 102. Both end portions of the series circuit constructed of the series-connected potentiometers 102 and 103 are commonly connected to a power supply E, and the connection portion of the serial circuit is grounded. Torque voltages are derived from sliding contactors 102a and 103a of the respective potentiometers 102 and 103 by supplying the current of the power supply thereto, and these torque voltages are inputted to an electronic control circuit 104 employed in a power steering apparatus. Then, each of torque voltages Vm and Vs appearing at each of input resistors Rm and Rs provided at the input terminal of this electric control circuit 104 is entered via A/D converters 105 and 106 to a microcomputer 107. In the microcomputer 107, a calculation is made of a motor current instruction value based upon the torque voltage value Vm of the main potentiometer 102, and a drift is detected based upon the torque voltage values Vm and Vs of both of the main potentiometer 102 and the sub-potentiometer 103.
As indicated by a solid line in FIG. 21, torque voltages representative of a mutual reverse phase characteristic (cross characteristic) are produced from the respective potentiometers 102 and 103 constructed in the above manner. The torque voltage Vm of the sliding contactor 102a and the torque voltage Vs of the sliding contactor 103a become the same value as the voltage value Vo when the input torque is zero. For instance, assuming now that the respective sliding contactors 102a and 103a are moved together toward the lower side in the circuit diagram of FIG. 20 by applying right steering torque thereto, the torque voltage Vm is decreased in a substantially linear form, whereas the torque voltage Vs is increased in a substantially linear form. On the other hand, assuming now that the respective sliding contactors 102a and 103a are moved together toward the upper side by applying left steering torque thereto, the torque voltage Vm is increased in a substantially linear form, whereas the torque voltage Vs is decreased in a substantially linear form. Then, the torque voltages Vm and Vs represent the same voltage values when the absolute values of the applied torque are equal to each other.
In the torque sensor having such an output characteristic, a drift in torque sensor outputs caused by the variations in the power supply voltage, and also the aging changes in the contact resistance values of the connector connected to this torque sensor is detected as follows:
For example, assuming now that the power supply voltage E applied to the torque sensor 101 is decreased due to the temperature changes, drifts appearing in the respective torque voltages Vm and Vs have small amounts when the output voltage is low, whereas drifts own large mounts when the output voltage is high, as indicated by a broken line of FIG. 21, namely these drifts are not constant. Therefore, first of all, a voltage Vo appearing at a neutral point, corresponding to an average value of the torque voltage Vm and the torque voltage Vs when there is no drift, is previously precalculated based on the following formula: EQU Vo=1/2(Vm+Vs) (1).
Then, with employment of the torque voltage Vmd of the main potentiometer containing the drift and the torque voltage Vsd of the sub-potentiometer containing the drift, a drift value .DELTA.Vd is calculated as an average value of deviation with respect to the voltage Vo appearing at the neutral point based on the following formula: EQU .DELTA.Vd=1/2(Vmd+Vsd)-Vo (2).
Next, an absolute value of this drift value .DELTA.Vd is calculated. This calculated absolute value is compared with a preset value. Then, an occurrence of this drift is detected by judging whether or not the calculated value is larger than the preset value.
However, in the above-described conventional drift detection circuit, the drift is detected based upon a difference between the torque voltages of the two systems constructed of the main potentiometer and the sub-potentiometer. The drift value "Vd" is given from the above-described formulae (1) and (2) as follows: EQU .DELTA.Vd=1/2(Vmd-Vm)+1/2(Vsd-Vs) (3).
As a consequence, the drift value .DELTA.Vd is defined by adding a half of the deviation component of the main potentiometer to a half of the deviation amount of the sub-potentiometer. For example, since the deviation amount of the sub-potentiometer becomes substantially zero at the left end of the characteristic diagram shown in FIG. 21, the drift value .DELTA.Vd to be calculated becomes only the 1/2 deviation amount of the main potentiometer.
As described above, since the drift calculated in the above-described prior art is detected as such a value smaller than the actual variation value of the power supply voltage, the drift detection sensitivity would be lowered. There is a problem that the drift could not be detected until the difference in the output signal voltages of the two signal paths becomes a certain large value. As a consequence, there is a risk that the drive current containing the drifts will flow through the electric motor until the drifts are detected, and thus the steering wheel would be self-steered. This may impede safety drive.
On the other hand, the torque-detection-value-abnormality detecting apparatus for detecting the abnormality of the torque detected value from the torque sensor is described in, for instance, Japanese Patent publication No. Hei. 6-9973. In this prior art, when the difference between the torque detected values outputted from the first displacement-to-electric signal converting unit and the second displacement-to-electric signal converting unit employed in the torque sensor is larger than or equal to a predetermined value, a judgement is made that the torque sensor is abnormal. At this time, the operations of the electric motor and the electromagnetic clutch of the electric power steering apparatus are stopped so as to maintain the vehicle under safe state.
However, in the above-explained conventional torque-detection-value-abnormality detecting apparatus, the difference value between the main torque detection value and the sub-torque detection value is compared with a preset value to judge the abnormality. When this predetermined value .DELTA.T is preset, in order to correctly judge the abnormality, it is desirable to preset the value by considering tolerance in the connector contact resistance values predictable during the manufacturing/assembling operations. Thus, for example, as shown in FIG. 22, in the case that there is the tolerance T.sub.OFF in the second displacement-to-electric signal converting unit on the sub-signal path side, since a larger preset value .DELTA.T is set by taking this tolerance T.sub.OFF into consideration and also by giving a clearance so as to stably judge the normal/abnormal conditions, there is a problem that precision in detecting the abnormality would be lowered.
Also, in the prior art described in the above-mentioned publication, as indicated in FIG. 23 for instance, when the connector contact resistance value of the signal line in the first displacement-to-electric signal converting unit on the main signal path side is increased to thereby vary the torque voltage value, the variation range (i.e., difference in torque detection values) near the neutral position where the applied steering torque is low would appear as a small value, as compared with the variation range where, for instance, the right steering torque is high. As described above, there are differences in the variation ranges every time the steering torque is applied. Thus, there is another problem. That is, it is difficult to keep the detection precision constant.